1. Field of the Invention
This invention relates generally to a method for making a stabilized rechargeable cell employing a lithium metal negative electrode in a molten salt electrolyte (MSE), particularly with an MSE of relatively low temperature.
2. The Prior Art
It is known to stabilize lithium electrodes in a secondary cell that employs a chloroaluminate-molten salt electrolyte system.
That is, lithium deposition and stripping is known employing EMIC/AlCl.sub.2 /LiCl (EMIC=1-ethyl-3-methylimidazolium chloride) room-temperature molten salt following addition of a proton source such as EMIHCl.sub.2 or triethanolamine-hydrogen chloride. However, the chloroaluminate system has several disadvantages which limits its use as an electrolyte for high energy density batteries: (1) the concentration of proton required for a stable lithium deposit is difficult to maintain for extended lengths of time and (2) the melt is not air stable and must be handled under inert conditions.
That is, if the cell is not properly sealed and leaks, the chloride in the melt will react with moisture in the air and form HCl, a corrosive gas.
Yet it would be desirable to utilize the advantages that low temperature molten salts offer (e.g., excellent electrochemical window, high conductivity and variable temperature range) but eliminate some of the previously mentioned problems for the chloroaluminate system.
By "low temperature" as used herein is meant temperatures below 100.degree. C., including room temperature of about 20.degree. C.
That is, it would be advantageous to use a lithium metal electrode in an MSE secondary or rechargeable cell for high voltage potential at, e.g. room temperature.
In the prior art relative to rechargeable cells having a molten salt electrolyte and a lithium electrode, one finds references such as U.S. Pat. Nos. 4,076,905 (1978) and 4,116,780 (1978) both to Sammells and U.S. Pat. No. 4,304,825 to Basu (1981). In each of these references, the lithium electrode is combined with another material, either as an alloy of silicon and boron or intercalated in graphite, with no suggestion of being able to employ a lithium metal electrode in a molten salt electrolyte for a rechargeable or secondary cell.
Accordingly there is a need and market for such a secondary cell that overcomes the above prior art shortcomings.
There has now been discovered such secondary cell wherein the lithium electrode can be charged and discharged repeatedly, while protecting the lithium electrode from reaction and erosion by the MSE, to gain advantage of the wide electrode chemical window attendant such cell system.